Steel laminations at one end of a dipole magnet for the Main Injector.

HANK
GLASS.The experiment is ready
and waiting. The researchers have calibrated each calorimeter crystal and
adjusted the high voltages on every wire in the tracking chambers. Every
one of the hundred thousand readout channels of the detector is primed to
record the swift passage of hadronic jets, electrons, muons, and other debris
ejected from a violent subatomic collision. All that the experimenters are
waiting for now is the arrival of high-energy protons and antiprotons, streaming
in from opposite directions to collide head-on in the center of the detector.

But how do the protons know where to go? They don't have
a road map. They can't hop on a bus. They can't stop a random pedestrian
and ask, "Hey, mister, do you know the way to CDF?"

The protons need magnets to show them the way.

The two main jobs of a particle accelerator are to raise
the energy of the particles and to steer them in the right direction. These
jobs are performed by different devices: RF cavities use electric fields
to accelerate the particles, while magnets use magnetic fields to steer
the particles. This article will explore some of the different kinds and
uses of magnets in accelerators.

Magnetic Personality

How are the magnets used at Fermilab different from the
magnets you use to stick your grocery list to the fridge? Obviously they're
bigger-a Main Injector dipole is 20 feet long and weighs 42,000 pounds,
making them impractical for kitchen use. But they're different in another
important way: unlike your kitchen magnets, in which the fields go around
the outside of the magnet, an accelerator magnet has its field on the inside.
Accelerator magnets have a pipe going through the middle of them where the
protons travel; in Fermilab's magnets, the field is directed into that pipe
to steer the particles, and very little field escapes outside the magnet.

Magnetic fields act in ways unlike any other force in nature.
In a familiar force, such as gravity, all objects are accelerated in the
same direction. We don't see some things fall up, some down, and others
sideways. Magnetism is more complicated. First, it only acts on electrically
charged particles; when humans, who are all electrically neutral, stand
next to a magnet, they don't feel much (unless of course they've got a steel
wrench in their pocket). Not only must the particle be charged, it must
also be moving: a proton sitting in the middle of a strong magnet will just
sit there and spin the time away, but another proton zipping in at high
speed will get kicked by the field. The force exerted by a magnetic field
on a moving particle not only depends on how fast it's going, but in which
direction: one particle, travelling at high speed in the direction of the
field (that is, along a magnetic line of force) will experience no force
at all, but another particle, moving at the same speed but at right angles
to the field, will be deflected by it.

Magnet Types

A simple type of magnet is called a dipole, and consists
of two poles. Magnetic lines of force emerge from one pole (North) and re-enter
the magnet at the other pole (South). In the space between the poles, where
the beam pipe resides, the field is nearly uniform. Magnet builders arrange
these dipoles around the circumference of a circle, and have all their fields
pointing straight up, which is just what is needed to get a beam of protons
to circulate around the circle in a clockwise direction. Antiprotons, having
negative charges, would circulate around these same magnets counterclockwise.

The dipoles are all Fermilab would need were it not for
the fact that a beam of protons is a disorderly bunch. They're not all moving
in exactly the same direction, but, instead, some want to drift sideways
while others want to move up or down, away from the plane of the ring. To
keep them in line, we need another type of magnet called a quadrupole, which
means a four-pole magnet (two North and two South poles). The field in this
type of magnet is zero at dead center, but grows linearly as you move further
away from the center. This means that a well-behaved proton moving along
the center, where it's supposed to go, will be left alone by the quadrupole.
But an unruly proton, wandering off the beam axis, will be pushed back towards
the center. The further away it is, the harder it gets pushed. This results
in a focusing of the beam of protons, similar to what a glass lens does
to a beam of light.